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Prof Sir John Beddington was the UK government’s chief scientific adviser between 2008 and 2013. He is a senior strategy adviser at the Oxford Martin School and professor of natural resource management at the University of Oxford.

Europe is currently considering a renewable energy directive that would raise the requirements to use renewable energy from a level of roughly 16% of final energy demand in 2015 to a level of 27-35% by 2030.

While this is a laudable target, policymakers do need to consider very carefully some potential unintended consequences of the rules that they are proposing. There is a real risk that these policies may even lead to a situation whereby global emissions accelerate.

The flaw is this: the directive will use an expansive classification of bioenergy products, allowing countries, factories and power plants to claim credit as renewable fuel for using trees harvested specifically for use in power plants and not merely residues and wastes

Burning wood by-products – that is, residues and production wastes – has been long practiced in Europe. This can benefit the climate thanks to a two-fold displacement effect.

First, energy is produced from biomass when it would otherwise have been produced by fossil-fuel energy, so significant emissions are avoided by keeping those fuels in the ground.

Additionally, the alternative fate of wood residues and production wastes would most likely be to rot in the forest or in landfills, emitting greenhouse gases as they did so. While the action of burning wood waste itself creates CO2, the net effect is beneficial because of these displacement effects.

Shaping the directive to allow trees themselves to be harvested for the sole purpose of energy means that several other science and policy complexities come into play.

First, the alternative fate of trees that are harvested directly for bioenergy, unlike the fate of wood residues and production wastes, is to remain standing as living biomass for decades, locking up their carbon for that period. If the trees are actively growing, they will also be actively absorbing CO2, and this same rule applies to small and large trees alike. If bioenergy uses trees or portions of trees that would otherwise be used as sawn timber or for paper products, then trees elsewhere must be harvested to replace those products.

For existing wood products, it is beneficial to engage in “sustainable management” of forests with sequential planting, harvest and regrowth periods, but even sustainable management does not avoid the “carbon debt” created by harvesting additional trees for energy. Several studies (for instance, Laganiere et al 2017 , Mitchell et al 2012) have calculated carbon payback periods for forest bioenergy under various management regimes at decades or even hundreds of years: yet it is now, in the early part of the 21st century, when we need to drastically reduce our carbon emissions.

Second, harvesting and burning trees is inefficient as a system for clean-energy production. Around a third of a tree is left behind (as roots and small branches) to decompose into the soil (a carbon-emitting process). Then, the lower burning temperature of wood and its greater carbon intensity means wood releases more carbon than fossil fuels per unit of energy generated (almost 4 times more than natural gas, and over 1.5x that of coal). Strategies to increase that efficiency, such as the formulation of wood pellets or co-firing of wood with coal, inevitably have the result of increasing associated emissions.

Third, there are implications for cross-border supply and demand of forest products. We have already seen how European renewable energy mandates and carbon accounting rules have led to the development of large numbers of wood pellet plants across the southeastern US and Canada, in order to fulfil European demand.

The scale of increased demand for forest bioenergy that this directive could produce cannot be over-emphasised. Since the last European directive on renewables in 2008, the growth of bioenergy has provided around half of renewables expansion.

To supply even a third of the additional renewable energy likely necessary by 2030, a fairly simple calculation from Europe’s reported annual wood harvest shows that at least an amount of wood equal to all of that harvest would be needed. That level is also roughly equal to the combined harvest in the US and Canada. With demand for other forest products, such as paper and pulp, burners of wood will have to look for other sources of supply.

Fourth – and as a potential consequence of the third point – there are potential risks for the incredibly valuable tropical forests that are not only valuable sources of biodiversity, but that also form vast carbon sinks and which are one of our best tools of defence against climate change.

The assumption that wood is inherently carbon neutral works as a kind of “reverse REDD” policy, sending a message to other countries that they can claim an economic return for the carbon value of their trees, but only if they cut them down. If Europe chooses this route, it seems likely that a consequence will be vastly greater forest removals, which still mostly come from natural forests globally.

The result of promoting a system of biomass electricity from dedicated tree harvesting, as calculated by a large number of studies from different research groups and governmental organisations, including the UK’s then-Department of Energy and Climate Change, will in all realistic scenarios be substantially more carbon in the air for decades regardless of the type of forest used and no matter how sustainably they are managed.

A reasonable estimate (based on Laganiere et al 2017, Mitchell et al 2012, DECC 2014 and our own analysis) might be that every kilowatt hour of wood at least doubles the emissions over a period of 30 years that might otherwise occur even if the alternative were fossil fuels.

If forest harvests supply one third of the additional renewable energy, the implication might therefore mean 6% or even more carbon in the atmosphere over that period compared to use of fossil fuels rather than at least 6% less that would come from using other renewables like solar or wind: a swing of 10% of Europe’s emissions.

One occasionally used argument from the bioenergy industry is that it often uses residues, including thinnings, that would otherwise be left in the forest to decompose. Evidence even from the American Forest and Paper Association shows that is not generally true. In fact, harvests may even occur of highly value valuable wetland forests. Yet the very fact that the industry can use residues and wastes is another reason the law should require them to do so.